Many industrial and commercial operations utilize filter apparatus for clarifying liquids. In dry cleaning operations, for example, solvents, such as perchloroethylene and trichlorethylene, are used in order to remove solid particles, such as lint, dirt, and chemical contaminants from soiled clothing. Since the solvents are relatively expensive, such commercial dry cleaning operations utilize filter apparatus in conjunction with the dry cleaning machine for removing the solids and chemical impurities from the dry cleaning solvents in order to permit reuse of the solvents.
Two different types of filter apparatus commonly are used in commercial and industrial dry cleaning operations. One type is a replaceable cartridge filter which contains a pleated paper-type filter medium for physically removing solid materials and a filter core of granulated activated charcoal for adsorbing chemical impurities. A disadvantage of cartridge filters is that as solid materials accumulate on and clog the filter, the efficiency of the filter is irreversibly impaired. The dry cleaning machine eventually must be shut down and the cartridge removed and replaced with a new one. Not only are the cartridges relatively expensive, but upon removal, they contain up to a gallon or more of solvent, which is wasted. Moreover, since the solvent is toxic, it is deemed a hazardous waste which necessitates governmental regulated disposal of the cartridge. The cost of the wasted solvent, together with the cost of disposal, typically approximates the initial cost of the cartridge filter.
Another type of filter apparatus commonly used in commercial and industrial dry cleaning operations is a reusable pre-coat filter, which includes mechanical filters made of stainless steel mesh or fine mesh synthetic fabric filter cloth such as nylon, polyester, or the like, and involves depositing filter-aid powder, such as diatomaceous earth, onto the filter to form a filter cake of the desired porosity. Pulverized activated carbon also can be mixed with the filter-aid for adsorbing chemical impurities in the solvent. After solids from the unclarified solvent accumulate on and clog the surface of the filter, the accumulated solids and filter-aid may be dislodged from and reapplied to the filter so as to again render the filter cake porous. As solids build up in the system after prolonged operation of the dry cleaning system, however, the solids and other contaminants ultimately must be separated from the solvent so as to permit reuse of the solvent with a new charge of filter-aid.
Heretofore, problems have been incurred in effectively removing the accumulated solids and filter cake from pre-coat filters without damage to the filter and without time-consuming delays in operation of the dry cleaning system. When brushes or scrapers have been utilized for physically scraping or brushing the accumulated solids and filter-aid from filter surfaces, such as fine mesh stainless steel screens, damage, puncturing, or undesirable wear to the screen can result. When more delicate filter surface materials are employed, such as fine mesh synthetic fabric filter cloth, physical brushing or scraping cannot be used. Efforts to remove the filter-aid and accumulated solids from the filters without brushing or scraping often do not result in complete removal of the accumulated solids and filter-aid. Residue remaining on the filter, and particularly slimy residue, tends to impede liquid flow through the filter which is necessary for reestablishing a uniform filter cake on the filter for subsequent operation.
Further problems have been incurred in the eventual removal of the solids from the system so as to permit reuse of the solvent with a new charge of filter-aid. Prior separation and removal methods employed for such purpose often have been inefficient and result in sludges with relatively high solvent contents, which create strong, unpleasant odors and necessitate disposal of the sludge as a hazardous waste material. In an effort to reduce the solvent content in sludges produced in distillation processes, it is known to introduce steam into the distillation vessel during the final stages of distillation, subsequently condense the water and solvent vapors generated, and then separate the condensed water and solvent. The disadvantage of such procedure, commonly referred to as a steam sweep, is that substantial quantities of water can be generated, which even after the separation step, can contain sufficiently high amounts of solvent as to render the liquid toxic and disposable only as a hazardous waste material.